The standard method of preparing monomers for the production of methyl silicone resins involves the following steps:
Methyl chloride is reacted with finely divided silicon and a copper catalyst at around 300.degree. C. to produce chloromethyl silanes. This reaction is known as the direct synthesis or Rochow synthesis, named after the chemist who discovered the process.
Next the chloromethyl silanes are fractionated by distillation to purify the desired intermediates.
The separated chloromethyl silanes are hydrolyzed with water to produce condensation products and byproduct hydrochloric acid.
The oligomers produced in the hydrolysis step are further purified by distillation. For example, dichlorodimethyl silane on hydrolysis forms the cyclic tetramer with a boiling point of 175.degree. C. The monofunctional compound chlorotrimethyl silane on hydrolysis forms hexamethyldisiloxane.
Finally, the purified oligomers are polymerized to the desired resins.
Although the above scheme represents a great improvement over earlier routes involving the use of Grignard reagents, there are fundamental drawbacks with the technology. First, purification of the chloromethyl silanes is difficult because trichloromethyl silane boils only 4.degree. C. lower than dichlorodimethyl silane. Distillation columns with up to 200 trays are required and even then reflux ratios approaching 100:1 are needed to obtain the desired purities.
Furthermore, the hydrolysis step, while using straightforward chemistry, is troublesome in execution. A highly corrosive byproduct, hydrochloric acid, is formed in the conversion. Expensive materials of construction are therefore necessary. Also, this waste acid stream must be treated or other means must be found to dispose of this byproduct.
The condensation products from the hydrolysis step likewise present challenges. Invariably, sludges are formed which reduce yields and make the purification of the cyclics more difficult. The cyclics distillation step requires a significant capital expenditure and adds considerably to the cost of utilities.
In order to circumvent the shortcomings of existing technology, numerous proposals have been made to prepare the methoxy derivatives from the chloromethyl silanes produced in the direct synthesis. The rationale for this approach is that the methoxy compounds are easier to separate than the chloro compounds. Moreover, the methoxymethyl silanes are excellent monomers for silicone resins; they readily polymerize to produce the desired high molecular weight polymers required in this application.
Accordingly, the hydrolysis step in the standard synthesis is replaced by a methanolysis reaction. Thus, chloromethyl silane is reacted with methyl alcohol to form methoxymethyl silane and hydrogen chloride. In some proposals methyl chloride instead of hydrogen chloride is formed. Regardless of the reaction conditions, however, it is almost impossible to prevent the formation of some hydrogen chloride. Furthermore this acid reacts with the methanol to form water which in turn reacts with chloromethyl silane to produce more hydrogen chloride. Immediately some of the silanes condense to form sludges.
To get around these difficulties, various suggestions have been made. Complicated arrangements for adding one reactant to the other have been outlined. Another approach is to employ a hydrogen chloride receptor. For example, ammonia and various amines have been mentioned for this use. Additional methods include blowing dry air or nitrogen through the reaction mixture, the use of vacuum, or the use of refluxing solvents. Because of the sensitivity of silanes to hydrolysis, however, none of the above proposals have proven to be entirely satisfactory.
Therefore, it is an object of the present invention to provide a silicone monomer process which overcomes the disadvantages of existing processes. It is an object to provide for the efficient separation of monomers in the required purities. A further object is to avoid the production of hydrogen chloride. Still another object is to produce monomers of the needed functionality such that they can be readily polymerized to the desired silicone resins.
These and other objectives, features and advantages of the invention will become apparent from the following description and the accompanying drawing FIG. 1.